The present invention relates to a method for exposing surface defects on steel blanks having a given hot working temperature. The term "steel blanks" as used herein is intended to cover billets, slabs, ingots, rods, wire and the like. The surface defects are made visible by a particular type of descaling and controlled cooling as disclosed herein.
It is known to make surface defects in steel blanks visible by first permitting the blanks to cool down to room temperature and then descaling the surface of the blanks, for example, by shot or sand blasting or by grinding. In another known method the blanks are subjected to a pickling operation, whereby the pickling liquid has normally a temperature of about 60.degree.C. Shot and sand blasting have the disadvantages that fine cracks and holes in the blank surface are closed by the impact of the shot pellets or the sand and thus become invisible. Pickling on the other hand changes the entire surface of the blank in such a manner that it becomes hard to discover small defects especially very narrow cracks and tiny holes. Besides, both methods, blasting and pickling, have the disadvantage that they may be performed only after the blanks have cooled down. Hence, it is not possible to take corrective steps in the manufacturing process, especially where a continuous manufacturing process is involved, such as extrusion casting or continuous casting. This is especially undesirable where the number of defects is high, because the descaling after the cooling may thus make it necessary that all blanks of an entire charge must be discarded, because of an accumulation of defects.
The rolling of coarse oxidation scale into the surface of the blanks is undesirable for several reasons, for example, such scale may later on damage any shaping tools which are employed in further machining the blanks. Such rolled in scale also reduces the quality of the product itself. Thus, it is well known to remove scale by means of water jets under high pressure prior to a hot working operation, such as drawing, piercing, or pressing. Reference is made in this connection to U.S. Pat. No. 2,394,514 granted on Feb. 5, 1946. Reference is made in this connection further to U.S. Pat. No. 1,898,809 granted on Feb. 21, 1933 where it is said that the application of water as an intermediate cleaning step will not interfere with the prompt reforming or reshaping of the heated and cleaned metal objects. However, since immediately after the hot working such as die shaping or pressing the oxidation continues to form new scale, the surface defects will not remain visible for an inspection subsequent to the hot working. Thus, contrary to the assumptions as, for example, stated in U.S. Pat. No. 1,898,809 that a descaling subsequent to the hot working is not necessary, it has been found that very minute surface defects require a descaling and controlled cooling subsequent to the hot working of the blanks as disclosed herein, especially where it is intended to keep the defects visible for longer periods of time.